I used a test rim with multiple vertical holes extending through the rim. I used wood dowel ‘slides’ (like a trombone) to vary the depth of the holes while the banjo was assembled. This way, I could test multiple depths and locations of these holes. Each hole acts as an integral tone chamber adding tones to modify the main chamber, a simple cylinder, of the pot.
After testing hundreds of combinations, I found that just 4 holes with depths based the chromatic scale of a 25 ½” string length worked best. They added nice full overtones without ‘wolf notes’ that can occur with too many tone chambers of similar sizes. The accuracy of drilling these holes is also critical to their functioning well.
Previously, I used 12-13 holes place between bracket holes. These started with a 2” deep hole and other decreased in depth using the fret scale formula of eighteenths of the scale length. Earlier, in the 1960’s, I built a hollow-walled banjo with 2 plys separated by vertical blocks of wood. There were many other reasons why this one didn’t sound like I dreamed that it would, but the concept stuck with me all these years.
When I met Mike Seeger, he told me that his favorite banjo was based on one by Kratzke that had holes drilled vertically in the rim. He said that he had no idea what size or how deep they were. I didn’t try to find out what he did because I wanted to follow my own instincts to develop my own version of this concept.
This model of the ‘chromatic’ rim is the result.
I think that if produces a rich tone over the whole range from bass to treble. There’s fullness of tones with sustain but plenty of snap and projection that I want in a banjo. I’ve found that using different combination of tone woods for the rim and tone rim yield slight variations to satisfy different tastes.